much like the Final fantasy 1 series paradox of monk to master is monk has higher defence than a master thus a monk will take 200 dmg from tiamat while the master will take 500 dmg

so sheryl is kinda screwed since compared to other characters is a bit weaker in combat but her abilities are praticlly good.
so kinda impossible to avoid her being a sissy in 1 on 1 combat, so even with the weapon shop bar full it doesnt mean her base stats are up to par with everyone else.

She starts with a nice life stat, but since she joins later her hp doesn't go up that much if you don't take the time to level. If you do level a bit you should easily get her level a fair bit above 100 which is enough to keep her alive - an auto-regen and "shield" items help greatly as well.

Phlebas the Phoenician, a fortnight dead, Forgot the cry of gulls, and the deep sea swell // And the profit and loss.
- - Gentile or Jew // O you who turn the wheel and look to windward, Consider Phlebas, who was once handsome and tall as you.
[T.S.Eliot: The Waste Land - IV. Death by Water]

Shield gear, and just kill things faster. If you put in enough def and have the shield item, you should be taking like 1-30 damage at max, regardless of enemy, including final boss. She was taking way less damage than cliff was without a shield gear, but he has the hp to spare.